The Paleoclimate of Central Texas

The climate of central Texas has undergone precipitation and temperature changes on millennial as well as annual time scales. In general, the region has undergone significant warming and drying since the last glacial maximum approximately 24,000 years before present. Mechanisms for these changes have ranged from millennial scale orbital cycles to annual sea surface temperatures. It is important to determine the magnitude of these changes and understand the underlying mechanisms for change. The extent to which scientists can identify and quantify these changes will assist in determining the usefulness of climate models.

24,000 to 14,000 Years Before the Present Day

• During this period, the climate was the coolest and wettest of the last 24,000 years.  There is a presence of low latitude ice sheets in North America. This is evidenced by:
• Fossil evidence indicates that large mammals once present in the region are now extinct (Lundelius, 1967; Graham, 1976)
• Animals once living in this region emigrated to regions that are cooler and wetter (Toomey et al, 1993)
• The remains of ground dwelling fauna suggest thicker soils and thus cooler conditions (Toomey et al, 1993)
• Pollen data from this time period suggests that trees which grown in cooler summer temperatures (~22° C) were living in the region (Potzger and Tharp, 1943, 1947, 1954; Bryant, 1969, 1977; Camper, 1991)
  Higher elevations once dominated by woodlands are now inhabited by desert vegetation (Spaulding et al, 1983).
• Alluvium deposited ca. 15,000 years before present also suggests that during the late Pleistocene conditions were cooler and wetter than the Holocene. This is inferred from a 13C value of -21‰, which indicates a large amount of trees and C3 grasses (40-50%) in the region (Nordt, 1994).
• Magnetic susceptibility (MS) analysis from Hall’s Cave, TX indicates the lowest value of the 19,000 years of analysis (Ellwood and Gose, 2006).
• 87Sr/86Sr ratios of fossil Hackberry aragonite and vole/pocket gopher tooth enamel suggest that 20,000 years before present there was a peak in soil thickness and as time increased soil depth decreased (Cooke et al, 2003).

14,000 to10,500 Years Before the Present Day

During this time period it is inferred that temperatures began to warm and glaciers began to recede to higher latitudes.  This resulted in warmer and drier conditions in central Texas as the jet stream was positioned further north thus limiting the amount of cold air brought to the region.  This is evidenced by:

• Disappearance of the masked shrew which prefers cooler temperatures and the appearance of the cotton rat which can live in temperatures of more than 24°C (Hall, 1981).
• Loss of the bog lemming (ca. 14,000 ybp) which indicates decreased effective moisture (Toomey et al, 1993)
  The appearance of a drier weather shrew and emigration of a shrew that requires wetter conditions (Toomey et al, 1993).
• There also appears to be an increase in grass pollen which suggests higher temperatures and less effective moisture versus the glacial period (Toomey et al, 1993).
• Alluvium deposited 11,000-8,000 years before present are inferred to be composed of 50-60% of C4 grasses and thus represents a shift to drier and warmer conditions (Nordt et al, 1994).
• Magnetic susceptibility data describe this as a time period of continued warming and drying (Ellwood and Gose, 2006).

10,500 to 5,000 Years Before the Present Day

The early Holocene is ushered in as the warming and drying trend of the previous 4,000 years continues as the earth begins a retreat from glacial conditions. This is evidenced by:

• Loss of vertebrate fauna with a preference for wetter conditions and the emergence of species that are adapted to drier conditions (Toomey, et al, 1993)
• Pollen records also indicate an increase in grass species beginning 10,500 years before present while 8700-6000 years before present pollen records indicate a larger abundance of taxa favoring more xeric conditions near Hinds Cave (Toomey et al, 1993; Bryant and Holloway, 1985).
• A site at Boriack Bog has a pollen record which indicates a gradual loss of trees and increase in grasses (Bryant and Holloway, 1985). Included in this is evidence of a shift of tree populations away from this region and into wetter regions (Bryant and Holloway, 1985).
• Archaeological evidence at Mustang Springs reveals that wells were dug around 6800 years before present, which are interpreted to be a response to drier conditions and a lowering of the water table (Metzler, 1991).
  Sedimentary records suggest “eolian sedimentation” between 6500-4500 years before present due to drier conditions (Holliday, 1989) and after 8000 years before present the rate of deposition of soil in Hall’s Cave doubled, clay deposition decreased and limestone clast deposition increased (Toomey et al, 1993).
• Vertebrate fossils indicate the disappearance of prairie dogs which need thicker soils in which to reside (Toomey et al, 1993).
• Sedimentary evidence from the Fort Hood alluvium deposited between 8000-5000 years before present indicates that C4 grasses comprise 65 to 70% of the fauna, which represents a warming and drying of the climate (Nordt et al, 1994).
• Magnetic susceptibility data from this time period indicate a general warming trend with a pronounced, extreme period of warming beginning around 8400 years before present and peaking at 8200 years before present (Ellwood and Gose, 2006).

5,000 to 2,500 Years Before the Present Day

It appears that sometime around 5000 years before present there was a peak in warmness/dryness and the time period from 5000-2500 years before present represents the driest/warmest climate of the Holocene. This is evidenced by:

• Regional extinction of taxa requiring larger amounts of moisture such as the pipistrelle bat and the woodland vole (Toomey et al, 1993).
• Represented at Schulze Cave by the absence or minimum of wet weather shrew and the abundance of a dry weather shrew population ca. 3800 years before present (Dalquest et al, 1969).
  Gastropods samples from Bering Sinkhole suggest the driest conditions to occur between ca. 4000 and 2700 years before present (Toomey et al, 1993).
• 13C values of snail shells reflect an abundance of C4 grasses and that around 5000 years before present was the warmest and driest of the Holocene (Goodfriend and Ellis, 2000).
• Magnetic susceptibility data from this time period point towards significant warming events ca. 4400 ybp, 3200 ybp, and 2900 ybp (Ellwood and Gose, 2006).

2,500 to 1,000 Years Before the Present Day

There was an apparent a return to cooler/wetter conditions ca. 2500 years before present which extends to ca. 1000 years before present. This is evidenced by:

• Fossil evidence indicates the return of the woodland vole and pipistrelle bat to the Hall’s cave area as well as a significant increase in the abundance of the least shrew (a wetter condition species) around 2500 years before present (Toomey et al, 1993).
• Pollen records indicate an increase in pine trees and grasses associated with cooler conditions or more moisture (Bryant and Holloway, 1985) and oak-woodland occurs in east-central Texas between 2500-1500 years before present (Holloway et al, 1987).
• Springs at Mustang Springs in the Southern High Plains began to discharge again around 2000 years before present (Meltzer, 1991).
• Increasingly negative average 13C values (-6.5‰) of snail shells, dating from 2700 to 1500 years before present, suggest that conditions were moving towards cooler/wetter (Goodfriend and Ellis, 2000).